JP2023124645A - carbon fiber electrode material - Google Patents

Abstract

To provide a carbon fiber electrode material that can be applied as a gas diffusion layer for a fuel cell to increase power generation efficiency by improving discharge characteristics (aeration characteristics) of generated water vapor in the fuel cell while maintaining cushioning properties required for the electrode material, and that can increase a contact area, lower a channel resistance, and lower an electrical resistance in a thickness direction in liquid cells such as redox flow cells and electrolyzers.SOLUTION: A carbon fiber electrode material is characterized in that at least one of a warp and a weft that are formed from a carbon fiber fabric and constitute the carbon fiber fabric is a spun yarn, and on at least one surface of the carbon fiber fabric, at a floating part of the spun yarn in a woven tissue, a raised fabric part given by some of a plurality of single fibers constituting the spun yarn being cut is formed, and the raised fabric part is extended obliquely with respect to the weft and the warp.SELECTED DRAWING: Figure 2

Description

TECHNICAL FIELD The present invention relates to a carbon fiber electrode material made of carbon fiber fabric, which can be used in fuel cells, redox flow batteries, electrolyzers, and the like.

A polymer electrolyte fuel cell (hereinafter referred to as a fuel cell or FC), which is the mainstream for home and vehicle use, consists of 1) platinum catalyst bonding layers (hereinafter referred to as CCM layers) on both sides of the polymer membrane, 2) A unit unit (hereinafter referred to as a cell) consisting of a gas diffusion layer (hereinafter referred to as GDL) that guides fuel gas and oxidant gas to the catalytic reaction zone, 3) a separator having gas introduction/discharge grooves, a sealing material, etc. is repeated. It is a laminated structure. A redox flow battery is a secondary battery in which an electrolyte membrane is sandwiched, an electrolytic solution is circulated between a positive electrode and a negative electrode, and charging and discharging are repeated by increasing and decreasing the number of charges of the metal ions. The use of carbon fiber fabrics for the gas diffusion layers of fuel cells, the electrodes of redox flow batteries, and the hydrogen electrodes of electrolyzers having almost the same structure as fuel cells has been studied.

First, the gas diffusion layer used in the fuel cell is generally a member formed in a thin sheet of 1 mm or less. It is required to have a function of smoothly supplying to the electrode catalyst layer. In addition, the basic functions of the gas diffusion layer include: 1) having a sufficiently low electrical resistance to extract electrical energy efficiently; 2) sufficient gas permeability to extract a large current; 3) have cushioning properties (elasticity) capable of absorbing thickness unevenness and changes in thickness of laminated members; 4) resist strong acidity and strong alkalinity; It is necessary to have corrosion resistance that can withstand even Further, electrodes for liquids in redox flow batteries and electrolyzers are required to have low electrical resistance in the thickness direction, low channel resistance in the planar direction, and a large wetted area.

Conventional gas diffusion layers have a paper-making structure (carbon paper) made mainly of carbon fibers, which has the problem of insufficient gas permeability and low cushioning properties (elasticity) to absorb unevenness in thickness. Ta. In addition, when high rigidity and cushioning properties are required for the properties of carbon paper, the basis weight increases, and the electrical resistance in the thickness direction of the gas diffusion layer as a whole also increases. As a result, the fuel cells themselves generate heat, and power generation efficiency decreases. On the other hand, if you try to reduce the basis weight of the carbon paper in order to reduce the electrical resistance, when the short cut carbon fiber is made into paper, the number of fibers bridging the blank part is reduced, so the rigidity is reduced and it is pressed against the CCM layer. The pressure becomes smaller and the contact resistance increases. As a result, power generation performance deteriorates. In the case of using carbon paper, there is a problem of poor drainage of generated water generated in a large amount in a high current range.

Therefore, for the purpose of maintaining a good balance between drainage performance and water retention performance in the cell of a polymer electrolyte fuel cell and ensuring a sufficient supply of gas to the polymer electrolyte membrane, A technique of intermittently distributing meshes of different sizes in a plane direction in a carbon cloth (carbon fiber fabric) has been disclosed (see, for example, Patent Document 1). However, even if a densely woven carbon cloth is used, the MPL layer (porous smooth layer), which makes good contact with the CCM, has unevenness. There is also the problem that the pressure required for contact cannot be secured and sufficient adhesion cannot be obtained.

Also, in order to solve these problems, a gas diffusion layer in which carbon cloth and carbon paper are laminated has been proposed (see, for example, Patent Document 2). However, in the gas diffusion layer in which the carbon paper and the carbon cloth are simply laminated, when the carbon paper and the carbon cloth are individually laminated in the assembly process of the membrane-electrode assembly, each of the carbon paper and the carbon cloth is CCM. Since the pressure is applied to a level that does not damage the layers, the contact resistance increases. Furthermore, each of the carbon paper and the carbon cloth requires many manufacturing processes, which poses a problem in terms of cost.

Japanese Patent Application Laid-Open No. 2003-173789 JP 2009-295509 A

Therefore, in the present invention, while maintaining the cushioning properties required for the electrode material, in the fuel cell, the gas diffusion layer for fuel cells that improves the discharge characteristics (ventilation characteristics) of water vapor generated in the fuel cell and increases the power generation efficiency. A carbon fiber electrode material that can be applied as a carbon fiber electrode material that can increase the contact area, reduce the flow resistance, and reduce the electrical resistance in the thickness direction in liquid cells such as redox flow batteries and electrolyzers. The task is to provide

The carbon fiber electrode material of the present invention is formed from a carbon fiber fabric, at least one of the warp and the weft constituting the carbon fiber fabric is a spun yarn, and at least one surface of the carbon fiber fabric has a woven In the floating portion of the spun yarn in the structure, a raised portion is formed by cutting some of the plurality of single fibers constituting the spun yarn, and the raised portion is formed on the weft and warp. On the other hand, it is characterized by extending in an oblique direction.

In the carbon fiber electrode material of the present invention, it is preferable that the raised portion is formed by cutting 25% or more and 50% or less of the number of single fibers constituting the spun yarn of the weft.

In the carbon fiber electrode material of the present invention, the spun yarn on which the raised portion is formed has a thin portion and a thick portion, and the thread width of the thin portion is the thick portion of the thread width. It is preferably 50% or more and 75% or less of the yarn width.

In the carbon fiber electrode material of the present invention, it is preferable that the raised portion is spanned over weft or warp yarns other than the spun yarn that is the source of cutting.

In the carbon fiber electrode material of the present invention, the carbon fiber fabric is preferably satin weave or mat weave.

In the carbon fiber electrode material of the present invention, it is preferable that the raised portion is bound to the suspended weft or warp with a binder containing conductive fine particles and fluororesin.

According to the present invention, the gas diffusion layer for a fuel cell increases the power generation efficiency by improving the discharge characteristics (ventilation characteristics) of water vapor generated in the fuel cell while maintaining the cushioning properties required for the electrode material. A carbon fiber electrode material that can be applied as a carbon fiber electrode material that can increase the contact area, reduce the flow resistance, and reduce the electrical resistance in the thickness direction in liquid cells such as redox flow batteries and electrolyzers. can be provided.

FIG. 1 is a schematic structural diagram illustrating formation of raised portions in a carbon fiber fabric. FIG. 2 is a schematic structural diagram explaining a state in which the raised portion is extended. 3 is a micrograph of the surface of the carbon fiber electrode material of Example 1. FIG. 4 is a diagram showing the results of surface roughness measurement in Example 1. FIG. 5 is a diagram showing the results of surface roughness measurement in Comparative Example 1. FIG.

Hereinafter, embodiments of the carbon fiber electrode material of the present invention will be described with reference to the drawings. However, the present invention is not limited or restricted to the following examples. It should be noted that the drawings referred to below are schematically described, and the dimensional ratios of the objects drawn in the drawings may differ from the dimensional ratios of the actual objects. The dimensional ratios of objects and the like may differ between drawings.

The carbon fiber electrode material of the present invention has a layer formed from carbon fiber fabric. A carbon fiber fabric is a carbon fiber fabric obtained by firing and carbonizing acrylic fibers. Of the warps and wefts forming the carbon fiber fabric, at least the wefts are spun yarns. A raised portion is formed by cutting a part of single fibers among a plurality of single fibers that constitute the spun yarn. FIG. 1 is a schematic structural diagram for explaining the formation of raised portions in a carbon fiber fabric, and shows, as an example, a satin weave in which three out of four wefts are floating. As shown in the figure, some of the single fibers are cut at the position indicated as "cutting point" in the floating portion of the weft (the portion where the weft is above the warp) in the weave structure of the carbon fiber fabric. Thus, a raised portion is formed. As shown in FIG. 2, the raised portion extends obliquely with respect to the weft and warp. The raised portion can be extended obliquely in this manner by aligning the raised portion in one direction with a brush or an air stream and laying down the fluff. By extending the raised portion in an oblique direction, the fibers straddle the grains of the carbon fiber fabric and cross between the warp yarns in a manner of a bridge. Therefore, it is possible to obtain a surface structure in which undulations are suppressed while having a woven structure. When such a carbon fiber fabric is used as the base material of the gas diffusion layer, it is possible to suppress the occurrence of depressions and openings when forming the microporous layer on the base material. Here, since it is a spun yarn, a few fluffs may extend diagonally in the weave, but this degree is not a preferable crosslinked state.

In the raised portion, it is preferable that 50% or more and 75% or less of the number of single fibers constituting the spun yarn of the weft remain without being cut. As the cutting ratio increases, the strength in the weft direction decreases. Therefore, from the viewpoint of maintaining the performance (tensile strength) as a carbon fiber electrode material, it is preferable that 50% or more of the carbon fibers remain uncut. In addition, if the cut rate is low and the raised portion is too small, the carbon fiber fabric cannot be sufficiently straddled, and there is a risk that the undulations will be suppressed insufficiently. preferably remaining. That is, it is preferable that the number of single fibers cut is 25% or more and 50% or less, and the gaps in the texture are covered.

The ratio of the number of single fibers remaining without being cut can be measured as follows. First, it can be confirmed by counting the number of uncut single fibers from a photomicrograph of the carbon fiber electrode material. Alternatively, instead of measuring with a micrograph, spun yarn is separated from the carbon fiber fabric, and in the yarn bundle, the average yarn width of 10 clearly thin parts is measured using a magnifying glass, and the 10 clearly thick parts are measured. It can be calculated by displaying % as a ratio to the average yarn width at one point. At this time, a fuzzy yarn that is clearly spun yarn and a yarn that is clearly separated from the yarn bundle because it is non-twisted yarn is ignored.

It is preferable that the raised portion is aligned at an angle of 30° or more and 60° or less with respect to the warp. The alignment angle is an acute angle such as θA or θB in FIG. Even when using a carbon fiber electrode material after forming the MPL layer, which will be described later, the condition of the raised portion can be determined by firing at a temperature equal to or higher than the decomposition temperature of the resin forming the MPL layer. Formation of such a raised portion can be performed, for example, by the following method. Sewing is sewn on both ends of a belt-shaped fabric made of extra-fine PAN spun yarn, which is the starting material of the carbon fiber fabric, and the selvage ends are reinforced. A needle-introducing roller is pressed against the cloth surface to cut and raise a part of the monofilaments of the weft that are slightly floating. The number of single fibers to be cut at that time can be adjusted by adjusting the tension of the cloth. As a raising device, KU-50S manufactured by Kanai Kikyu Kogyo Co., Ltd. can be selected. The needle-implanting roller is a roller in which needles are radially planted. Further, the raised single fibers are fluffed in a certain direction with a brush or an airbrush so that the raised single fibers are aligned at an angle of 30° to 60° with respect to the warp. Next, this fabric is sandwiched between heat-resistant and smooth flat plates with a predetermined shim to secure an appropriate gap (preferably a gap of 1 to 2 times the thickness of the fabric), and the fabric is sandwiched in a fluff-down state to make it flameproof. and carbonization firing. Here, it is also important to lay down the raised single fibers by stroking them with a soft brush for painting, etc., and at the same time remove floating fibers so that short fibers do not come loose during actual use. .

It is preferable that the raised portion is stretched over weft or warp yarns other than the spun yarn that is the source of cutting, and the stretched weft yarns or warp yarns are bound by a binder containing conductive fine particles and fluororesin. is more preferred. MPL ink can be used as the binder. The MPL ink is a material for forming a microporous layer, and is generally an ink in which conductive fine particles such as carbonaceous powder, fluororesin particles as a binder thereof, and a surfactant are dispersed in water. After applying the MPL ink, it is dried and sintered to bind the weft and warp threads. In the case of a resin for the microporous layer containing conductive fine particles, for example, polytetrafluoroethylene resin (PTFE), it is cured at 370° C. for 5 minutes to bind. In addition, for redox flow battery electrode applications, for example, polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), etc., can be bound with a fluororesin containing fine particles that disappear when heated under the above conditions. , the efficiency is improved because the specific surface area is increased, which is preferable. The particle diameter of the fine particles is preferably 5 μm or less, more preferably 2 μm or less.

In this way, the fluff-bound single fibers (raised portion) are bound to the warp and weft in a state that covers the weave space, so that when the electrolyte membrane is sandwiched between the two electrodes in actual use, In addition, the adhesion between the MPL layer and the electrolyte membrane in the weave space portion of the carbon fiber electrode material is ensured, and a good contact state between the two is ensured. On the other hand, on the separator side opposite to the MPL layer, if the woven fabric of the carbon fiber electrode material bites into the grooves of the separator, the flow paths will be reduced and the drainage will be poor. However, in the fuel cell gas diffusion layer using the carbon fiber electrode material of the present invention in which fibers (raised portions) bridging the texture are present, the fluff-bound fibers (raised portions) are bound to the warp and weft, as described above. Since it is attached and reinforced, the amount of bite into the separator groove is reduced, and air permeability and drainage are maintained.

In order for the raised portion to sufficiently straddle the weave of the carbon fiber fabric and to ensure that the raised portion spans over other yarns, the length of the raised portion needs to be longer than the width of the mesh of the fabric. Therefore, the weave structure of the carbon fiber fabric is, for example, a satin weave in which one out of three or one out of four wefts floats, and by cutting the floated wefts, the length of the raised portion is secured. can do.

In FIG. 1, the satin weave in which three of the four wefts are floating is shown as an example of the cutting point, but the present invention is not limited to this. Some of the single fibers may be cut at the floating portion of the weft, and the formed raised portion may extend in an oblique direction, but a woven structure having two or more floating wefts is preferable. . In addition to the above, more specifically, 5-ply 2-ply satin weave, 5-ply 3-ply satin weave, 7-ply 2-ply satin weave, 8-ply 3-ply satin weave, and two or more yarns are put together and plain weave. A mat weave or the like is preferable. If the number of floats is too large, the restraint of the warp threads is weak, and the weave tends to become irregular, such as misalignment. Therefore, it is desirable that the floating number be up to five.

The carbon fiber electrode material of the present invention is a woven fabric, and has a structure in which the warp yarns are bridged by a raised single yarn (raised single fiber) of the weft, and one end of the raised single yarn (raised single fiber) is a weft It is characterized by a twisted structure. As a result, it is possible to secure a good conductor through which electrons can move even in the intermediate portion of the weave, and to promote the electrochemical reaction. In addition, it can be said that the conductive resistance in the fibers constituting the carbon fiber electrode material of the present application can be lower in contact resistance than in materials such as carbon paper in which short fibers are bound and fixed with a binder. . In addition, if it is a woven fabric, by using a weaving method such as a plain double weave, a fine linear concave-convex structure in the direction along the thread is formed, which is not easily formed with paper such as carbon paper or non-woven fabric. You can also This structure can be used as a channel for liquid or gas, and when sandwiched between separators, the channel resistance of the fluid can be reduced, which is preferable.

In the above, a woven fabric using spun yarn as at least the weft was explained, but in the present invention, the spun yarn may be used only for the warp instead of the weft, It is also possible to cut some of the single fibers of both the warp and the warp.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples.

[Example 1]
1. Preparation of Gas Diffusion Layer for Fuel Cell (1) Starting Fiber Fabric As the starting fiber fabric, an acrylic spun yarn fabric with a cover factor of 959 in terms of denier and a satin weave in which 4 out of 5 wefts float was woven. The warp is a 100-meter count yarn that is untwisted with vanishing fiber (PVA), and the weft yarn is a 200-meter count untwisted yarn. There were 70 warp threads per inch. After weaving, the lost fibers were washed off with warm water to obtain a woven fabric composed of substantially non-twisted yarn.

(2) Formation of raised portion The floating wefts of the obtained acrylic spun yarn fabric are subjected to a fluffing machine (KU-50S, manufactured by Kanai Jitsu Kogyo Co., Ltd.) so that the fluff is not partially broken, and the fluff is observed with a magnifying glass. As a setting condition, the single fiber is partially cut to form fluff (raised part), and the fluff is laid down with a brush so that the fluff is at 45 degrees to the warp, and the fluff is laid down. was made. Here, the basis weight of the fabric after fluffing was 94% of the basis weight of the fabric without fluffing.

(3) Carbon fiber fabric production A shim that secures a gap 1.5 times the thickness of the obtained fabric after fluffing is placed on the outer periphery, and the fabric after fluffing is sandwiched between heat-resistant plates, and flameproofing is performed. A carbon fiber electrode material, which is a fluff-covered carbon fiber fabric, was obtained by performing a carbonization treatment. Here, the thickness of the woven fabric is the value obtained when a flat-pressed 1 cm diameter end of a Mitutoyo Digimatic Indicator is pressed from above. FIG. 3 shows a surface micrograph of the fluffed surface of the obtained carbon fiber electrode material. The weft yarn of the obtained fuzz-bound carbon fiber fabric was separated, and the average yarn width of 10 apparently thin portions and the average of 10 clearly thick portions were measured using a magnifying glass in the yarn bundle. When the thread width was calculated, the average thread width of the thin portion was 66% of the average thread width of the thick portion. It can be estimated that 34% of the number of single filaments of this weft spun yarn is cut. The warp pitch of the obtained carbon fiber electrode material was 280 μm.

(4) MPL ink application (MPL layer formation)
A water-soluble MPL ink (binder) containing carbon black and carbon nanotube particles as conductive fine particles and PTFE resin as a fluororesin was prepared. Then, this MPL ink was applied to the obtained carbon fiber electrode material by a transfer method so that the coating weight was 11 g/m ² , and baked at 370° C. for 5 minutes. It was confirmed that the raised portions were bound in the carbon fiber electrode material after sintering.

In the present application, the term "basis weight" has the same meaning as "a unit representing the mass per unit area of woolen fabric, etc., and the number of grams per ^1m2 " in Japanese Industrial Standards (JIS) L02028.

(5) Surface roughness measurement After applying MPL ink and applying pressure to the baked carbon fiber electrode material from zero to 200 N / cm ² 5 times, the MPL ink application side (MPL layer side) was measured with a digital microscope (manufactured by Keyence Corporation, VHX700) having a non-contact surface roughness function. FIG. 4 shows the results of surface roughness measurement. The surface roughness in the warp cross-sectional direction was within a maximum of 5 μm at a period of 280 μm pitch, and the amount of dents did not affect the power generation performance when used as a gas diffusion layer for a fuel cell. Although not shown, a part of the single fiber of the spun yarn is cut from the separator side opposite to the MPL layer and extended in a direction oblique to the weaving yarn direction to cover the surface of the MPL layer at the center of the weave. observed to cover.

[Comparative Example 1]
As the starting fiber fabric, a plain weave acrylic spun yarn fabric made of the same spun yarn as in Example 1 and having a cover factor of 959 in terms of denier was used, and the raised portion was not formed in the same manner as in Example 1. , to obtain a carbon fiber electrode material. The weft yarn of the obtained carbon fiber fabric was separated, and the average yarn width of 10 clearly thin portions and the average yarn width of 10 clearly thick portions were measured using a magnifying glass in the yarn bundle. was calculated, the average thread width of the thin portion was 87% of the average thread width of the thick portion. This value was within the range of spun yarn variation that occurs in a normal process without a cutting process. The warp pitch of the obtained carbon fiber electrode material was 280 μm.

FIG. 5 shows the results of measuring the surface roughness of the MPL layer side after applying and baking the MPL ink. The surface roughness in the warp cross-sectional direction was as high as 12 μm at a period of 280 μm pitch. This is a value that exceeds the standard that causes insufficient pressing when used as a gas diffusion layer for a fuel cell. It can be seen that in the case of the plain weave without forming the raised portion, the unevenness of the surface was not improved after the formation of the MPL layer.

In the carbon fiber electrode material of Example 1, the napped portion extends obliquely, thereby increasing the specific surface area and increasing the rigidity of the space in the central portion of the texture. Therefore, when forming the MPL layer (applying the MPL ink), the space in the central portion of the texture is less likely to be recessed. When the carbon fiber electrode material having the MPL layer formed thereon is sandwiched between the electrolyte membrane and the separator, the surface roughness of the MPL layer in the perpendicular cross-section of the warp yarns generates periodic unevenness of the warp pitch. If there is a fiber (raised part) that straddles the weave like a fluff-bound cloth, the fiber acts as a reinforcing fiber, and the deformation of the surface dents is a deformation amount (within 6 μm) that does not greatly affect the power generation performance of the fuel cell. ), and the adhesion is ensured. On the other hand, when the cloth is not covered with fluff and there is no reinforcing material across the weave, the recess is about twice as large as in Comparative Example 1, resulting in poor contact.

As for the woven structure, satin weave is generally more prone to unevenness than plain weave when the MPL surface is pressed. This is because the satin weave has a structure in which the bridging distance of the weaving yarn is long, so that the surface tends to be uneven. However, as described above, in the satin carbon fiber fabric in which the fluff is bound, after the formation of the MPL layer, the unevenness is improved compared to the plain weave of Comparative Example 1, and the fuzz bridging effect of Example 1 is It can be said that it has been shown that there is enough.

In the above, the carbon fiber electrode material of the present invention has been described by exemplifying the results of confirming the characteristics as a fuel cell gas diffusion layer. However, the application of the carbon fiber electrode material of the present invention is not limited to gas diffusion layers for fuel cells, and can be suitably used in liquid cells such as redox flow batteries and electrolyzers. In the form used in redox flow batteries and electrolyzers, the electrolyte membrane is sandwiched between positive and negative electrodes using the carbon fiber electrode material of the present invention, and is sandwiched between separators that secure fluid flow paths. can be done. This electrode also has the same requirements as the gas diffusion layer for fuel cells, such as low electrical resistance in the thickness direction, low flow path resistance, stable adhesion to the electrolyte membrane, and acid resistance. be.

Claims (6)

Formed from carbon fiber fabric,
At least one of the warp and the weft constituting the carbon fiber fabric is a spun yarn,
On at least one surface of the carbon fiber fabric, a raised portion is formed by cutting some single fibers out of a plurality of single fibers constituting the spun yarn at floating portions of the spun yarn in the weave structure. cage,
The carbon fiber electrode material, wherein the raised portion extends obliquely with respect to the weft and warp. 2. The carbon fiber electrode material according to claim 1, wherein the raised portion is obtained by cutting 25% or more and 50% or less of the number of single fibers constituting the spun yarn of the weft. The spun yarn on which the raised portion is formed has a narrow portion and a thick portion, and the width of the narrow portion is 50% to 75% of the width of the thick portion. The carbon fiber electrode material of claim 1, wherein: The carbon fiber electrode material according to any one of claims 1 to 3, wherein the raised portion is spanned over wefts or warps other than the spun yarn from which the material is cut. The carbon fiber electrode material according to any one of claims 1 to 4, wherein the carbon fiber fabric is satin weave or mat weave. The carbon fiber electrode material according to any one of claims 1 to 5, wherein the raised portion is bound with the suspended weft or warp and a binder containing conductive fine particles and fluororesin.

Images